1. Field of The Invention
The invention relates to the optical variable attenuator used in optical fiber communication system and optical network technology, and particularly to the dual fiber collimator for use therewith.
2. The Prior Art
As well known, optic attenuators are widely used in the optical transmission system and the optical network, and especially in the optical wavelength division multiplexing (WDM) network, which closely relates to "Project of Information Highway". The fiber optic attenuator is a passive optical component that is intended to reduce the optical power propagating in the fiber, and may be categorized to either a fixed or a variable attenuator, wherein the variable attenuator is deemed as an optimal one. The motor driven optical variable attenuator (OVA) is a new kind of device used in the WDM network, which can optionally control the receiving power to be at one certain level, so the performance of the network can be greatly improved with an expected value. Though OVA is deemed very usefully, there is less patents relates thereto. For example, U.S. Pat. No. 5,745,634 ('634) uses two lenses to design the optics coupling system and uses the normal DC motor to control motion, thus resulting in at least two disadvantages including unstability of the performance and the expensive price. In '634, the mechanical system thereof uses two gears to transfer the non-precision motion to the precision motion whereby it is uneasy to improve the tuning speed thereof.
Therefore, it is desired to have an optical variable attenuator with simple structures thereof and high reliable precise performance. Anyhow, the application Ser. No. 09/174,367 filed Oct. 15, 1998, now U.S. Pat. No. 6,144,794, discloses using a step motor with the lead screw and the corresponding nut to transform the rotation movement to the linear movement from the mechanical system viewpoint thereof whereby a neutral density (ND) filter can be linearly moved between two opposite collimators for performing a variable attenuator form the optical system viewpoint thereof.
It should be understood that oscillation is derived from interference coming from the two faces of the ND filter. Suppose that the reflection rates of such two faces of the filter are 99% and 1%, respectively, the transverse area of the light beam is large enough as shown in FIG. 2 wherein there is oscillation in the attenuation spectrum. Moreover, the return loss of a high quality OVA should be designed as high as 55 dB. Thus, in the two-collimator system of the copending application Ser. No. 09/174,367, the ND filter should be placed with a declination to purposely enlarge the return loss. While unfortunately this declination arrangement also enlarges the undesired insertion loss. As practiced in the aforementioned copending application Ser. No. 09/174,367, a wedge glass is introduced to provided a designed declination thereby resulting in not only the required 55 dB return loss but also the diffraction for restricting the resonance or the insertion loss thereof. Understandably, if the second collimator can not precisely and correctly received the expected light beam from the first collimator, the attenuation effect will be jeopardized.
Additionally, there are also some disadvantages in mechanism of the copending application. Because the slide used therein is a very expensive device and its mass is fairly large. Oppositely, the step motor can only afford limited torque, so that the tuning speed can not be very high; otherwise, the motor will lose steps, and cause trouble in setting of attenuation and result in bad repeatability performance. By using the slide, the turning speed more than 500 ms is not allowed for the whole attenuation range, while without the slide, the high precision motion direction might be worse.
Therefore, corresponding to the aforementioned optical viewpoint, an object of the invention is to provide a variable attenuator which uses a wedge type filter to cooperate with a dual-fiber collimator to provide declination thereof and precisely transmit the light beam therebetween, thus maintaining the return loss as high as 55 dB while without oscillation or significant insertion loss thereof from the optical viewpoint.
Corresponding to the aforementioned mechanical viewpoint, another object of the invention is to provide a variable attenuator with simplified structures which may meet the high turning speed of the operation requirements from the mechanical viewpoint.
Yet a third object of the invention is to provide the variable attenuator with a low polarization dependent loss (PDL). It should be noted that since the attenuation designedly changes 20 dB along the 4 mm length of the filter, the filtering coating on the ND filter forms a very tiny wedge layer on the glass plate and makes the ND filter a polarization dependent device. Generally, the PDL of such a ND filter is more than 0.3 dB. This value is deemed too large and is desired to be lowered to be less than 0.1 dB.